1. Field of the Present Invention
The present invention generally relates to an onboard radar device and a program of controlling the onboard radar device.
Priority is claimed on Japanese Patent Application No. 2009-252654, filed Nov. 4, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
A technique of launching an electromagnetic wave of a milli-wave (mm) band to measure the distance from vehicles or obstacles around a user's vehicle, the relative speed, or the like has been known as a onboard radar, which is disclosed in Japanese Unexamined Patent Application, First Publication, No. 2006-275840, for example.
FIG. 22 is a schematic diagram illustrating a ground propagation model to describe a propagation of an electromagnetic wave performed by the onboard radar in accordance with the conventional art. In FIG. 22, an electromagnetic wave launched from a transmitting antenna 10 of the onboard radar propagates on a route r1, is reflected from a reflection point 12 such as a front vehicle and an obstacle at a position of a distance R1 from the transmitting antenna 10, propagates on the route r1, and is input to a receiving antenna 11 disposed at a position substantially closer to the transmitting antenna 10 or used in common with the transmitting antenna 10. The onboard radar measures the distance to the reflection point 12 such as a front vehicle and an obstacle, and the relative speed by a phase difference of the received electromagnetic wave.
In the above-described onboard radar, actually, there is also a propagation route of reflecting from a reflection point 11 (ground surface) through a route r2 and following a route r3, in addition to the case where the electromagnetic wave propagates on the route r1, as shown in the terrestrial propagation model shown in FIG. 22. For this reason, electromagnetic waves propagating from a plurality of routes are synthesized at the reflection point 12, the synthesized wave reflected from the reflection point 12 propagates again on the route of propagating on the routes r3 and r2 in addition to the route r1, and the electromagnetic wave synthesized at least twice or more is input to the receiving antenna 11. The synthesized electromagnetic wave is input to the receiving antenna 11. The distance to the reflection point 11 is d1, the distance from the reflection point 11 to the reflection point 12 is d2, and the distance from the transmitting antenna 10 and the receiving antenna 11 to the reflection point 12 is R1. The height of the transmitting antenna 10 and the receiving antenna 11 is h1, the height of the reflection point 12 is h2, and generally, h1 is equal to h2. The incident angle of r2 and r3 with respect to the ground surface is θ2.
Hereinafter, attenuation characteristics of the electromagnetic wave between the antenna and the reflection point will be described using the terrestrial propagation model shown in FIG. 22. FIG. 23 is a diagram illustrating an example of calculation of free-space propagation attenuation and propagation attenuation including an indirect wave. Generally, an electromagnetic wave is exponentially attenuated according to the distance between transmission and reception R1. For example, an electromagnetic wave (direct wave) propagating on the route r1 is exponentially attenuated as R=R1 in the formula represented by the numerical expression (1), and has attenuation characteristics shown by the broken line in FIG. 23.
                              P          free                =                                            P              t                        ⁢                          G              t                        ⁢                          G              r                        ⁢                          λ              2                        ⁢            σ                                                              (                                  4                  ⁢                  π                                )                            3                        ⁢                          R              4                                                          (        1        )            
In the case of including an electromagnetic wave (indirect wave) propagating on the routes r2 and r3, an element of r4 (r2+r3=r4) is included in the formula represented by the numerical expression (2). As a result, the attenuation characteristics fluctuate up and down, “canceling” parts and “strengthening” parts like faulting ridge portions and valley portions appear along a trace (broken line) represented by the numerical expression (1), and become the attenuation characteristics shown by the solid line in FIG. 23.Er=[e−jβr1(1+R1DT(θ1)e−jβ((r2+r3)-r1))][R2e−jβr1(1+R1DR(θ1)e−jβ((r2+r3)−r1))]=[e−jβr1(1+R1DT(θ1)e−jβ(r 4−r1))][R2e−jβr1(1+R1DR(θ1)e−β(r4−r1))]wherer4=r2+r3  (2)
The direct wave that is the electromagnetic wave transmitted through the route r1 and the indirect wave that is the electromagnetic wave transmitted through the route r2 and the route r3 have different propagating distances, and cause a phase difference and an amplitude difference at a destination that is the receiving antenna 11 of the onboard radar. A phenomenon of deteriorating the reception signal by the synthesis of the direct wave and the reflection wave in which the phase difference and the amplitude difference occur is called road surface multi-path fading.
In the milli-wave band onboard radar, road surface multi-path fading is one of the important factors causing measurement disturbance. When considering road surface multi-path fading in the onboard radar, the terrestrial propagation model shown in FIG. 22 is used. Hereinafter, in the onboard radar, the influence of road surface multi-path fading will be considered with reference to the power attenuation characteristics observed (with respect to the propagation route r1) by the radar or the receiving antenna 11.
In road surface multi-path fading of the milli-wave band onboard radar, there are several restrictive conditions different from those of the general communication. That is:
a) the transmission and receiving antennas are installed at relatively very low positions in height with respect to the distance between transmission and reception,
b) in the detection range (several tens of m to one hundred and several tens of m), the incident angle θ2 (see FIG. 22) with respect to the ground surface is substantially 80° or more.
The restrictive conditions are large factors affected by road surface multi-path fading.
FIG. 24 is a concept diagram illustrating the relation of power attenuation with respect to the heights of the antenna and the reflection point and the distance between transmission and reception in the terrestrial propagation model. In this simulation for calculation, the road surface is a road surface formed of concrete or the like, and the reflection wall is a completely flat conductor.
As shown in FIG. 24, when the height h1 of the antenna and the height h2 of the reflection point are hA, the attenuation characteristics are as shown by the broken line. When the height h1 of the antenna and the height h2 of the reflection point are hB used for the onboard radar, the attenuation characteristics are as shown by the solid line. That is, when the heights h1 and h2 of the antenna and the reflection point are very low hB used for the onboard radar with respect to the distance between transmission and reception, the difference in propagation distance between both routes of the direct wave and the indirect wave is small as compared with the case of hA. Accordingly, very wide “strengthening” parts and “canceling” parts occur as described above. As the distance between transmission and reception R1 gets larger (farther), the power attenuation of the “canceling” parts gets larger and the span tends to be wider.
More specifically, at parts surrounded by ovals, the power attenuation is less than the minimum detection precision shown by the chain double dashed line, and it is impossible to detect the reception electric wave. Particularly, at a position where the distance between transmission and reception is about RA, the width is a wide span and thus the detection is impossible.
FIG. 25 is a schematic diagram illustrating relation of the incident angle θ2 and the reflection coefficient with respect to the heights of the antenna and the reflection point and the distance between transmission and reception in the terrestrial propagation model. As illustrated in FIG. 25, the longer the distance R1 becomes, the incident angle θ2 becomes large and the reflection coefficient of the ground becomes large and the reflection becomes a total reflection approximately.
That is, when the heights h1 and h2 of the antenna and the reflection point are hB used for the onboard radar, the incident angle θ2 and reflection coefficient rapidly get larger as the distance between transmission and reception gets larger as compared with the case of hA. Accordingly, when the distance between transmission and reception R1 is larger than RB, the incident electric wave is reflected substantially without loss. For this reason, at the “canceling” parts (valley parts) shown in FIG. 24, it can be seen that the reception power tends to be substantially 0 (mW). That is, the onboard radar in which the heights h1 and h2 of the antenna and the reflection point are relatively low are greatly affected by road surface multi-path fading.
As described above, in the onboard radar, parts in which the chance of causing measurement disturbance is high are the “canceling” positions corresponding to the valley portions of the power attenuation characteristics. When the reception power value is less than the minimum detection precision of the onboard radar, the reflection wave signal cannot be detected. Thus, there is a problem that the object (vehicles, obstacles, etc.) at the corresponding distance cannot be detected. In addition, under the conditions for using the onboard radar, in the distance between transmission and reception of several tens of m to one hundred and several tens of m requiring the detection, the amplitude span is wide, and the amount of decrease in level is large. Accordingly, this influence is a very important problem in the onboard radar.